1. Field of the Invention
This invention relates in general to the field of information storage, and more particularly to correcting an asymmetric signal resulting from the use of magneto-resistive (MR) transducing heads in hard disk drives (HDD), and more particularly in HDD read channels.
2. Description of the Related Art
FIG. 10 shows a schematic representation of a typical HDD, including at least one disk 1006 having a magnetic medium for storing information, a spindle, a motor 1007, and a controller 1008 for controlling disk rotational speed, a transducing head 1005 (for reading and writing data), a servo actuator assembly including a positioning controller for positioning the head over the appropriate disk track 1004, and data channels 1003 for transmitting data to and from the disk. The transducing head 1005 reads data from and writes data to the disk in data blocks having either fixed or variable length. A data block comprises a preamble (for acquiring timing signals), timing bits, a position error field, address bits, data bits, and error correction bits. Data blocks are recorded in sectors in concentric tracks. A track may comprise several sectors. The number of sectors may depend on the radial location of the track on the disk.
Data channels on an HDD transmit and receive data via a communication medium. HDD and communication systems can utilize similar techniques to encode data for storage and retrieval or for transmission and reception wherein data is encoded into a form in which it may be easily manipulated. Most modern applications of such systems encode data as numeric or digital information, wherein discrete numeric values are used to represent user data.
The storage or communication media do not directly manipulate such digital data. Rather, these media store or transmit analog signals representative of the digital data. For example, encoded digital information may be represented as magnetic flux changes stored in the magnetic media of a hard drive. The data embodied in the medium are then received or retrieved to decode the signals and reproduce the encoded digital data. A read channel is a circuit that reads or receives the encoded data and reproduces the original digital signal. In general, a read channel includes a transducer component that senses the analog signal and digital processing components that detect sequences of changes in the signal that represent encoded digital data. For example, a read channel used in a magnetic storage device includes a transducer head that senses the magnetic flux changes and produces a continuous analog signal that must then be detected and decoded. In general, certain peaks in the continuous waveform represent the encoded digital data.
Transducing heads may be either inductive or magneto-resistive (MR). An MR head is a device whose resistance varies with the applied magnetic field. In this regard, the head is capable of converting magnetic field variations produced by data into a time varying voltage or current in an electrical circuit. MR heads offer many advantages over other types of magnetic transducers and, consequently, are increasingly being used in HDDs. For example, MR heads are more sensitive than other types of read heads, such as thin film heads, and produce a stronger read signal. Also, MR heads have a better frequency response than other types of heads which use inductive coils as a sensing means. In addition, the read signal produced by an MR head is relatively insensitive to the relative velocity between the head and the medium, than is the case with other types of heads, because it is the level of the applied magnetic field which is sensed by an MR head and not the rate of change of magnetic flux lines through a coil. This is an advantage in systems where head/medium velocity may vary over a significant range. Lastly, because MR heads are not capable of writing data on a magnetic medium, magnetic data storage systems using MR read heads must include a separate write head thereby making it possible to optimize the read head and the write head for their particular functions.
A disadvantage of an MR read head is that it produces a signal with an asymmetric amplitude. For example, the asymmetric nature of the MR head may cause the signal produced by the read head reading data to be different than the magnetic signal applied to the write head for writing data. To overcome this problem, a bias current may be applied to the head to move the quiescent operating point of the head to be more asymmetric of the resistance characteristic. However, due to variation in the microstructure and magnetic properties of the recording medium and other manufacturing variables, biasing cannot completely remove the asymmetric aspect of the signal.
One approach is to use a filter having varying tap weights to change the shape of the waveform. Another variation is having a digital filter differentiate the waveform into sample and recreate an equalized waveform. Both these approaches are relatively complex.
Another approach provides circuitry to detect the polarity of an asymmetric signal and to add or subtract a shift voltage to adjust for the asymmetry.
A disadvantage of these approaches is that the corrective circuitry is dependent upon layout and process parameters that are difficult to control. Therefore, a need exists to address the asymmetric signal effects produced by such MR heads by providing circuitry to overcome the problems and limitations of the prior art.